The invention is related to an electric actuator comprising a housing which contains a screw mechanism and an electric motor which is driveably connected to the screw mechanism, said screw mechanism providing a linear movement in response to a rotational movement of the electric motor and comprising a screw and a nut which is supported with respect to the housing, a sensor being provided for measuring a quantity related to the rotational movement and/or the linear movement of the screw mechanism.
Such electric actuator is known from WO-A-9403301. The electric motor of this actuator rotates a screw mechanism, which exerts a clamping force on the brake pads of a disc brake. In order to provide a fine tuning of the brake force, it is important to control the rotation of the screw mechanism accurately. Also, it is desired to control the screw mechanism for compensating brake pad wear which occurs in service in a disc brake.
The object of the invention is therefore to provide an electric actuator of the type described before, which allows a full control of the screw mechanism for all kinds of driving conditions and purposes, e.g. anti-theft. This object is achieved by means of a control unit for influencing the rotational and/or the linear movement of the screw mechanism on the basis of signals from the sensor.
As the control unit is continuously fed with information about the position and number of revolutions of the screw mechanism, it will always be updated so as to control the screw mechanism in the correct way.
The sensor may have various locations. In an actuator comprising at least one bearing for supporting the screw mechanism with respect to the housing, the sensor may be connected to at least one of the bearing rings. Also, the sensor may be connected to the screw, the nut or the housing.
In case the rolling elements are accommodated in a cage, the sensor may be connected to at least the cage.
For an embodiment having a roller spindle, the rollers of which are accommodated in a roller cage, the sensor may be connected to the roller cage.
The sensor itself may take several forms as well. For instance, the sensor is an optical sensor or a magnetic position encoder, e.g. comprises a pulse ring and a part which are rotatable with respect to each other, one of the pulse ring and the other part being immovable with respect to the housing.
As mentioned before, the actuator according to the invention is preferably used in a brake calliper for a disc brake, said actuator having an electric actuator as described before, the actuator having a housing which contains a screw mechanism and an electric motor which is driveably connected to the screw mechanism, said screw mechanism providing a linear movement in response to a rotational movement of the electric motor and comprising a screw and a nut one of which is supported with respect to the housing, a yoke onto which the housing is connected, and a pair of brake pads, one of which is connected to a fixed part of the yoke, and the other of which is connected to the screw or nut of the screw mechanism.
Here as well, a control unit may be provided for having a control unit for influencing the rotational and/or the linear movement of the screw mechanism on the basis of signals from the sensor.
This sensor may serve basic functions, such as giving information about wear compensation, brake force feedback (ABS) and maintenance indication. Additionally, monitoring functions for traction control, for vehicle dynamics and anti-theft are possible.
During its lifetime the brake pads wear out and therefore become thinner. This means that the roller screw has to compensate for the abrasion, which can be up to 30 mm. However, in order to guarantee safe operation, the distance between pads and brake disc always has to be maintained at approximately 0.2 mm. This can be done with the roller screw encoder since a number of pulses represents a certain distance. During braking, the brake pad will be pressed towards the disc with a certain force. When the brake is released, the force decreases and when it moves below a certain minimum level, when the pads are still touching the disc, the pulse counter is set to zero. Now the electric motor is turned further backwards until the proper number of pulses has been counted. In case the resolution of the sensor is 400 pulses per rotation, for a roller screw lead of 1 mm, this represents 1*0.2*400=80 pulses. In this way, calibration takes place at each brake operation. An advantage for the roller screw is that the spot where the most severe forces are applied, is gradually shifting over its full length, in discrete steps of {fraction (1/400)}th of a rotation.
In order to provide such wear compensation option, the control unit may comprise a counter for counting the number of revolutions over which the screw mechanism is rotated from a rest position to a full brake position, a memory comprising a fixed number of revolutions representing a maximum desired number of rotations from the rest position to the full brake position, a comparator for comparing the actual number of revolutions and the maximum desired number of revolutions, and resetting the rest position in case the actual number of revolutions exceeds the maximum desired number of revolutions.
Brake pad replacement can also be monitored in the brake calliper according to the invention.
In this respect, it is also possible to provide a control unit comprising a second counter for counting the total number of revolutions of the screw mechanism from a start position with unworn brake pads, up to the actual full brake position, a second memory comprising a maximum allowable number of revolutions, and a comparator for establishing whether the total number of revolutions exceeds the maximum allowable number of revolutions and for generating a warning signal indicating that maintenance is required in case the total number of revolutions exceeds the maximum allowable number of revolutions.
Furthermore, the control unit may comprise a third memory containing a set of brake characteristics data giving the brake force as a function of the number of revolutions for establishing the actual brake force on the basis of the actual number of rotations.
In the calliper described before, the displacement of the brake pads relative to each other is obtained by means of the screw mechanism. Such mechanism is fit for providing fairly large displacements, and for high loads. In a high duty cycle environment, preferably a roller spindle is used. A relatively large amount of the total travel of the screw mechanism is consumed by the flexibility of e.g. a calliper itself and by compensation displacements for accommodating for brake pad wear.
Only after removing the play as caused by structural flexibility, the brake pads start to deliver a significant braking action. The final phase of application of the brake pads onto the disc brake requires a significant torque, which might be disadvantageous having regard to installed motor power and screw thread wear.
In this respect, an improvement may be obtained in an embodiment wherein the screw mechanism engages a piezoelectric material actuation member.
In this embodiment, the screw mechanism is actuated up to a certain holding torque, for eliminating the slack or play. Subsequently, the piezoelectric material actuation member is actuated for obtaining the final braking displacement.
Preferably, the screw has a bore which opens out at the side of the associated brake pad, which bore contains a series of piezoelectric or magnetostrictive elements supported at one end at the bottom of the bore, and at the other end resting against a head which is connected to said brake pad.
The invention is furthermore related to a method for controlling a disc brake. Said method comprises the steps of counting the actual number of revolutions from a rest position to a full brake position, comparing said actual number of revolutions with a maximum desired number of revolutions, and resetting the rest position in case the actual number of revolutions exceeds the maximum desired number of revolutions for the purpose of compensating brake pad wear, as well as the step of establishing the difference between the actual number revolutions and the maximum desired number of revolutions, and using this difference for establishing a reset rest position upon resetting said rest position.
Moreover, the method for controlling the brake calliper may comprise the steps of counting the actual number of revolutions of the screw mechanism from a start position with unworn brake pads, up to the actual full brake position, comparing said actual number of revolutions with a maximum allowable number of revolutions, and generating a warning signal in case the total number of revolutions exceeds the maximum allowable number of revolutions for maintenance indication.